Magnesium lactate fermentation process

ABSTRACT

A fermentation process for producing magnesium lactate from a carbon source including the steps of: providing a fermentation medium including a fermentable carbon source in a fermentation reactor; fermenting the fermentation medium by a lactic acid producing microorganism in the presence of an alkaline magnesium salt to provide a fermentation broth including magnesium lactate; and recovering solid magnesium lactate from the magnesium lactate containing fermentation broth, wherein during at least 40% of the operating time of the fermentation process, the concentration of solid magnesium lactate in the fermentation broth is maintained in the range of 5-40 vol. %, calculated as solid magnesium lactate crystals on the total of the fermentation broth. The process allows stable operation at high productivity, in combination with efficient product separation.

The present application is a continuation application of U.S. patentapplication Ser. No. 16/092,378 filed Oct. 9, 2018, which in turn is aU.S. national stage application of PCT/EP2017/058547 filed Apr. 10,2017. Each of these prior applications is incorporated herein byreference in its entirety.

The present invention relates to the production of magnesium lactate viafermentation.

Magnesium lactate has many applications, one of which is a sourcematerial for lactic acid. Lactic acid may be used in numerousapplications such as the preservation of food and the preparation ofbiodegradable polymers. In some of these applications the quality of thestarting lactic acid is of utmost importance. For instance, in theproduction of lactide and polylactic acid it is desirable to start witha lactic acid with high stereochemical purity. Further, the presence ofimpurities in the starting lactic acid may result in undesirableracemisation of lactic acid moieties leading to a lactide and apolylactic acid product of lower quality.

The increasing demand for high quality products together with the needto achieve production costs compatible with the commodities market, makeit essential to be able to reduce the costs of the starting materialsfor the production of lactic acid while at the same time notcompromising the quality.

Lactic acid is often manufactured via fermentation of carbohydrates bymicroorganisms. To keep the pH of the reaction medium at a value wheremicroorganism grows well, an alkaline salt is often added duringfermentation, to compensate for the decrease in pH caused by theformation of lactic acid. This results in the formation of a lactatesalt. Where an alkaline magnesium salt is used, e.g., a (hydr)oxide orcarbonate of magnesium, magnesium lactate will be formed. Fermentationprocesses wherein magnesium lactate is formed, also indicated asmagnesium lactate fermentations, are known in the art.

For example, NL288829 describes a continuous fermentation process formanufacturing lactic acid where a magnesium salt or zinc salt is addedduring fermentation, to cause the formation of insoluble magnesiumlactate or zinc lactate, which is removed from the fermentation medium.

US2010/0323416 also describes a fermentation process to form acarboxylic acid, with addition of a magnesium salt.

WO2013160352 describes a fermentation process for the manufacture of,among many other products, magnesium lactate. The process encompasses asolid product removal step comprising a hydrocyclone and a solid/liquidseparation step.

Yong Wang et al., Efficient magnesium lactate production with in situproduct removal by crystallization, BioResource Technology, Vol. 198, 26Sep. 2015, pp. 658-663, describes a magnesium lactate fermentationwherein magnesium lactate is removed during the fermentation. It isindicated that removal of solid product during the fermentation shouldbe carried out at a magnesium lactate concentration of 140 g/l. Amagnesium lactate concentration of 140 g/l corresponds to a crystalconcentration of 5 vol. % at 42° C. In FIG. 3 of this document theproduct concentration in the fermentation is kept between 70 and 150 g/lduring the fermentation (with a startup period of 25 hours before avalue of 70 g/l is attained). As crystal formation starts at aconcentration of 110 g/l, this means that for a substantial part of thefermentation there are no solid crystals present at all and otherwiseonly at low concentrations.

To achieve the goal of a source for lactic acid which allows reductionof costs without compromising on product quality there is need in theart for a magnesium lactate fermentation process which allows stableoperation at high productivity, in combination with efficient productseparation. The present invention provides such a process.

The present invention pertains to a fermentation process for producingmagnesium lactate from a carbon source comprising the steps of

-   -   providing a fermentation medium comprising a fermentable carbon        source in a fermentation reactor,    -   fermenting the fermentation medium by means of a lactic acid        producing microorganism in the presence of an alkaline magnesium        salt to provide a fermentation broth comprising magnesium        lactate, and    -   recovering solid magnesium lactate from the magnesium lactate        containing fermentation broth,    -   wherein during at least 40% of the operating time of the        fermentation process, the concentration of solid magnesium        lactate in the fermentation broth is maintained in the range of        5-40 vol. %, calculated as solid magnesium lactate crystals on        the total of the fermentation broth.

It has been found that by ensuring that the concentration of solidmagnesium lactate in the fermentation broth is maintained in the rangeof 5-40 vol. % during at least 40% of the operating time, a process isobtained which combines a high yield with efficient product separationproperties. Efficient product separation properties translate into goodprocess stability, but also into high product quality, since it allows agood separation of the magnesium lactate from contaminants.

More specifically, it has been found that when the concentration ofsolid magnesium lactate is too high during a substantial part of theprocess, the productivity of the process decreases. Not wishing to bebound by theory it is believed that this may be caused by the presenceof the magnesium lactate fermentation product somehow influencing thefermentation process, e.g., by influencing the water activity in thesystem, and/or by somehow influencing the microorganism. This is incontrast with conventional wisdom, which teaches that solid fermentationproducts do not influence fermentation.

On the other hand it has been found that if the magnesium lactateconcentration is too low during a substantial part of the process, theseparation of solid magnesium lactate from the fermentation broth ismore difficult.

It may be preferred for the concentration of solid magnesium lactate inthe fermentation broth to be at least 10 vol. %, because it has beenfound that a higher concentration of solid magnesium lactate results ina product with improved properties, including improved filtrationproperties, which makes the product easier to wash. It may be preferredfor the concentration of solid magnesium lactate in the fermentationbroth to be in the range of 10-35 vol. % during the stipulated part ofthe operating time, in particular in the range of 10-30 vol. %, in someembodiments in the range of 10-25 vol. %. It may be more preferred forthe solid magnesium lactate in the fermentation broth to be in the rangeof 15-40 vol. %, in particular 15-35 vol. %, during the stipulated partof the operating time, more in particular in the range of 15-30 vol. %,in some embodiments in the range of 15-25 vol. %. In some embodiments itmay be preferred for the concentration of solid magnesium lactate in thefermentation broth to be in the range of 20-40 vol. %, in particular20-35 vol. % during the stipulated part of the operating time, more inparticular in the range of 20-30 vol. %, in some embodiments in therange of 20-25 vol. %.

The concentration of solid magnesium lactate in the fermentation brothis determined in accordance with the following procedure: A 1 mlhomogeneous sample is taken from the fermentation broth using anEppendorf tube. The sample is centrifuged for 2 minutes at 1300 rpm. Thevolume percentage of the solid layer is determined visually.

This solid layer comprises both solid magnesium lactate and biomass. Tocompensate for the amount of biomass, the amount of biomass may bedetermined separately by methods known in the art, e.g., by determiningthe optical density at 600 nm of a fermentation broth sample from whichcrystals have been removed by diluting it to 5 vol. % in a solution of0.5N EDTA adjusted to pH 8 with KOH, and comparing it with the OD600 nmof standard biomass solutions.

The volume percentage of solid magnesium lactate can then be determinedby subtracting the volume percentage of biomass from the percentageobtained in the centrifuge procedure described above.

The starting point for the operating time of the fermentation process isthe point in time when all medium components have been provided to thereactor, the fermentation medium has been brought to fermentationconditions, such as the selected pH and temperature, and themicroorganism has been provided to the reactor. At that point in timeall conditions have been met for the fermentation to begin.

The end point for the operating time of the fermentation process is thepoint in time when product formation has essentially stopped, i.e., whenthe production in g/l·h. is below 10% of the maximum value of productionin g/l·h during the process. This will generally be when the carbonsource has been depleted.

The total operating time of the process according to the invention mayvary within wide ranges. For commercial operation a suitable minimumoperating time is 10 hours. If the operating time is below this value,the period of time (in hours) during which the concentration ofmagnesium lactate is within the stipulated range will be so short thatmeaningful commercial operation may be difficult to achieve. It may bepreferred for the total operating time of the process according to theinvention to be at least 24 hours, in particular at least 48 hours. Themaximum number of hours is not critical. As described elsewhere herein,for a continuous process the total operating time may in principle beindefinite. A value of 2 years may be mentioned as a general maximum.

The concentration of solid magnesium lactate in the fermentation brothwill generally not be in the stipulated vol. % range during the entireoperating time. For example, at the beginning of the fermentation, itmay be that there is no magnesium lactate present in the medium. Uponstart-up of the fermentation, magnesium lactate is formed, which willfirst be in the dissolved state. Upon the formation of more magnesiumlactate, the fermentation medium will become saturated with magnesiumlactate, and solid magnesium lactate crystals will begin to form. Itwill then take some time for the value of 10 vol. % to be reached.

On the other hand, near the end of the fermentation, when the provisionof carbon source is stopped, it can be desirable to let the fermentationrun without further product removal, which may result in theconcentration of solid magnesium lactate in the fermentation broth tobecome higher than 40 vol. %. Further, especially when product removalis intermittent, it may be that the concentration of solid magnesiumlactate reaches a value of above 40 vol. % during the process at somepoints in time.

The percentage of the operating time during which the concentration ofsolid magnesium lactate is maintained in the ranges specified above, istherefore dependent on the amount of time taken up by the startup phaseand the end phase described above, in relationship to the time periodbetween such startup and end phase. Therefore, the longer the operatingtime is, the higher the percentage of operating time can be during whichthe concentration of solid magnesium lactate is in the stipulated range.

It is preferred that during at least 60% of the operating time of thefermentation process, the concentration of solid magnesium lactate inthe fermentation broth is maintained in the stipulated range, preferablyduring at least 70% of the operating time, more preferably during atleast 80% of the operating time, in some cases during at least 90% ofthe operating time.

The process according to the invention may be a batch process, afed-batch process, or a continuous process.

In one embodiment, the fermentation process according to the inventionis a batch process. Within the present specification a batch process isdefined as a process wherein the carbon source is provided to thefermentation reactor at the beginning of the reaction, and no(substantial portions of) carbon source are provided during the process.

In one embodiment, the fermentation process according to the inventionis a fed-batch process. Within the present specification a fed-batchprocess is a process wherein at least the carbon source is provided tothe fermentation reactor at the beginning of the reaction and during thereaction, which process has a predetermined end point beyond whichfermentation cannot be continued due to, e.g., the built-up ofimpurities.

In one embodiment, the fermentation process according to the inventionis a continuous fermentation process. Within the context of the presentspecification a continuous fermentation process is a process wherein atleast the carbon source is provided to the fermentation reactor at thebeginning of the reaction and during the reaction, wherein the processdoes not have a predetermined end point. In general, the total volume ofthe fermentation medium is kept more or less constant. This means that,in view of the addition of carbon source during the fermentation whichresults in an increase in the volume of the fermentation medium, contentwill be removed during the fermentation, in this case in the form ofsolid magnesium lactate, optionally in combination with some liquidfermentation medium. In principle, a continuous fermentation can runindefinitely, although it will at some point in time be discontinued forunit maintenance. The concepts of batch fermentation, fed-batchfermentation, and continuous fermentation are known to the skilledperson.

Conventionally in batch fermentations and fed-batch fermentations, thefermentation is continued until the carbon source has been depleted tosuch an extent that fermentation stops. The concentration of magnesiumlactate in the fermentation broth is determined by the amount of carbonsource, and can increase to very high values, e.g., of the order of 50vol. %, calculated on the total of the fermentation broth. These veryhigh concentrations can be obtained because magnesium lactate is presentin solid form.

In the present invention, however, the process will be carried out insuch a manner by appropriate recovery of solid magnesium lactate fromthe fermentation broth that the concentration of solid magnesium lactateis kept in the stipulated ranges during the specified part of theoperating time.

In one embodiment of the present invention, the fermentation process isa continuous fermentation process. In a continuous fermentation processcarbon source and other compounds are added during the fermentationprocess, so that the process can in principle run indefinitely. Incontinuous fermentation processes, intermittent product removal willtake place, to ensure sufficient space in the reaction vessel for thefermentation to continue. In the magnesium lactate fermentationsdescribed in literature, e.g., in NL288829, no information is providedon the magnesium lactate concentration in the fermentation broth atwhich product removal is carried out. In contrast, in the presentinvention it has been found that, contrary to expectations, by keepingthe solid magnesium lactate concentration within a specified rangeduring a specified part of the operating time a fermentation process isobtained which combines a product which shows good separation propertieswith a high volumetric productivity. The use of continuous fermentationis a preferred embodiment of the present invention, especially where theconcentration of solid magnesium lactate is kept in the stipulated rangeduring at least 70% of the operating time, more preferably during atleast 80% of the operating time, still more preferably during at least90% of the operating time.

The concentration of solid magnesium lactate in the fermentation brothis regulated be recovering solid magnesium lactate therefrom.

Recovery of solid magnesium lactate will generally be carried out duringthe process. It can be done in manners known in the art, e.g., by thesteps of withdrawing fermentation broth comprising solid magnesiumlactate from the fermentation reactor, and removing solid magnesiumlactate from the fermentation broth. The removal of solid magnesiumlactate from the fermentation broth can be done by methods known in theart, e.g., via filtration, centrifugation, decantation, or combinationsthereof.

In one embodiment the fermentation broth from which solid magnesiumlactate has been removed is recycled in part or in its entirety to thefermentation reactor. This may be attractive to restore biomass from thefermentation broth to the fermentation reactor.

The removal of magnesium lactate can be carried out in discrete steps inan intermittent fashion, but also in a continuous manner. Continuousmagnesium lactate removal is considered preferred, because it allowsaccurate control of the amount of magnesium lactate present in thefermentation broth. It can also efficiently be integrated in acontinuous fermentation process, which is a preferred embodiment of thepresent invention.

In general, if a step is carried out in which fermentation brothcomprising solid magnesium lactate is withdrawn from the reactor, thevolume of fermentation broth withdrawn in an individual step is at most40 vol. % of the fermentation medium present in the reactor. Higherpercentages will make it difficult to maintain the concentration ofsolid magnesium lactate within the specified range. It may be preferredif the volume of fermentation broth withdrawn in an individual step isat most 30 vol. % of the fermentation medium present in the reactor, inparticular at most 20 vol. %, more in particular at most 10 vol. %.

The process according to the invention also encompasses the steps ofproviding a fermentation medium comprising a fermentable carbon sourcein a fermentation reactor, and fermenting the fermentation medium bymeans of a lactic acid producing microorganism in the presence of analkaline magnesium salt to provide a fermentation broth comprisingmagnesium lactate. These steps are generally known to the person skilledin the art. They will be elucidated below for background purposes.

In the process according to the invention a fermentation mediumcomprising a fermentable carbon source is provided in a fermentationreactor. The term “fermentable carbon source” as used herein refers tocarbohydrates which can be fermented by a lactic acid producingmicroorganism. Examples of fermentable carbon sources are C5 sugars, C6sugars, oligomers thereof (e.g. dimeric C12 sugars) and/or polymersthereof, but also compounds like glycerol. By C5 sugars and C6 sugars ismeant saccharides with 5 and 6 carbon atoms, respectively, and by C12sugars it is meant saccharides with 12 carbon atoms (e.g. adisaccharide). The type of fermentable carbon source that a specificmicroorganism is able to ferment may vary and typically depends on thelactic acid-producing microorganism used. Examples of common sugarsfermentable by lactic acid producing microorganisms may include C5sugars such as arabinose, xylose and ribose; C6 sugars such as glucose,fructose, galactose, rhamnose and mannose; and C12 sugars such assucrose, maltose and isomaltose. It is within the scope of the skilledperson to select a suitable combination of carbon source andmicroorganism based on his common general knowledge.

The concentration of the carbon source in the reaction medium willdepend on the nature of the carbon source, the nature of themicroorganism, and the further fermentation conditions. It is within thescope of the skilled person to select a suitable concentration here.

The fermentation medium may be provided by combining additionalnutrients with the carbon source and water. The additional nutrients maybe added in any order and in solid form, as solutions or as suspensions(e.g. in water). Suitable nutrients for use in fermentation tomanufacture lactic acid or lactate salts are known in the art. Theadditional nutrients may be selected from at least one of, for instance,mineral salts (e.g. a source of mineral nitrogen, phosphate, sulfur andtrace elements such as zinc, magnesium, calcium, manganese, potassium,sodium, boric, iron, cobalt, copper, molybdenum, nickel, aluminum etc.)and a source of organic nitrogen (e.g. yeast autolysates andhydrolysates, plant protein hydrolysates, animal protein hydrolysates,and soluble by-products from steeping wheat or maize). Such organicnitrogen sources generally provide nitrogen in the form of, e.g., freeamino acids, oligopeptides, peptides, vitamins and traces of enzymecofactors. Such organic nitrogen sources further may also be addedindividually and/or in pure form.

The pH of the fermentation medium may be adjusted to a pH suitable forfermentation with the microorganism of choice, prior to inoculation.Generally, the pH may be adjusted to a pH from about 2.0 to about 8.0,in particular from about 4.0 to about 7.5. Depending on the initial pHof the fermentation medium, adjusting the pH may be performed byaddition of a base (e.g. an alkaline magnesium salt) or an acid (e.g.H2SO4) The fermentation medium is fermented by means of a lactic acidproducing microorganism in the presence of an alkaline magnesium salt toprovide a fermentation broth containing magnesium lactate. Thefermentation is generally performed by incubating the fermentationmedium with the microorganism at a suitable temperature for a suitableperiod of time.

During fermentation, magnesium lactate will precipitate in solid form.Whether or not precipitation of magnesium lactate occurs will depend onthe concentration of fermentable carbohydrates in the fermentationmedium, the fermentation temperature, the concentration of otherconstituents of the fermentation medium, the magnesium lactateconcentration and the dilution factor of the added alkaline magnesiumsalt.

Suitable lactic acid producing microorganisms are known in the art andmay include bacteria, fungi and yeasts, and may be selected frommicroorganisms that are (a) homolactic lactic acid producers or (b)heterofermentative microorganisms which produce lactic acid. Themicroorganisms may be genetically engineered to produce or overproducelactic acid. Examples of such microorganisms include, but are notlimited to, bacterial species of the genera Lactobacillus, Leuconostoc,Pediococcus, Lactococcus, Streptococcus, Aerococcus, Carnobacterium,Enterococcus, Oenococcus, Sporolactobacillus, Tetragenococcus,Vagococcus, Weissella, Bacillus (including Bacillus coagulans, Bacilluslicheniformis, Bacillus smithii, Bacillus thermolactis and Bacillusthermoamylovorans), Geobacillus (including Geobacillusstearothermophilus and Geobacillus thermoglucosidans),Caldicellulosiruptor (including Caldicellulosiruptor saccharolyticus),Clostridium (including Clostridium thermocellum), Thermoanaerobacterium(including Thermoanaerobacterium saccharolyticum), Thermoanaerobacterand Escherichia (including Escherichia coli), and fungal and yeastspecies from the genera Saccharomyces (including Saccharomyescerevisiae), Kluyveromyces (including Kluyveromyces lactis andKluyveromyces marxianus), Issatchenkia (including Issatchenkiaorientalis), Pichia (including Pichia stipitis), Candida (includingCandida boidinii, Candida magnolia, Candida methanosorbosa, Candidasonorensis and Candida utilis) and Rhizopus (including Rhizopusarrhizus, Rhizopus microspores and Rhizopus oryzae). Bacterial generathat are of particular interest are Lactobacillus, Bacillus (includingBacillus coagulans, Bacillus licheniformis, Bacillus smithii, Bacillusthermolactis and Bacillus thermoamylovorans), Geobacillus (includingGeobacillus stearothermophilus and Geobacillus thermoglucosidans) andEscherichia (including Escherichia coli).

Additionally or alternatively, preferred bacterial species are thosethat display optimal growth at a pH in the range of about 6 to about 8.

The incubation temperature may depend on the microorganism used. Forexample, the optimum temperature to be used may be established byanalyzing the activity of the fermentation microorganism under differenttemperature conditions. Generally, the temperature may be within therange from about 20 to about 80° C., in particular within the range fromabout 25 to about 70° C., and more in particular within the range fromabout 30 to about 60° C.

The alkaline magnesium salt added to the fermentation medium is used toneutralize the lactic acid excreted by the microorganisms duringfermentation generating a magnesium lactate salt. A drop in pH below acritical value, depending on the microorganism used in the process,could damage the metabolic process of the microorganism and bring thefermentation process to a stop. The pH is generally adjusted duringfermentation to be from about 2.0 to about 8.0, in particular from about4.0 to about 7.5. Adjusting the pH may be performed by controlling thepH of the fermentation medium and by addition of appropriate amounts ofbase when necessary. The alkaline magnesium salt may be selected from,for instance, at least one of MgO, Mg(0H)2, MgC03 and Mg(HC03)2. Thealkaline magnesium salt may contain minor amounts of other cations.

The magnesium lactate obtained by the process according to the inventionmay be processed as desired. It can be subjected to intermediatepurification steps in manners known in the art, e.g., viarecrystallisation, resulting in purified magnesium lactate.

The magnesium lactate can, e.g., by converted to lactic acid. This canbe done by various methods, including an ion exchange method, e.g. byuse of an ion exchange column or electrodialysis, or acidification usinga strong inorganic acid (e.g. sulfuric acid, HCl or HNO3) to provide amixture of lactic acid and a magnesium salt in an aqueous medium. Thismixture can subsequently be subjected to a lactic acid/magnesium saltseparation step, resulting in the formation of lactic acid and aseparate magnesium salt.

The separation step can be carried out by methods known in the art.Where the magnesium salt is in solid form, e.g., where sulphuric acid isused in the acidification step, the lactic acid/magnesium saltseparation step can be in the form of a solid/liquid separation step, inwhich the solid magnesium salt is removed, resulting in the formation ofan aqueous lactic acid solution.

Where the magnesium salt is present in the mixture as a dissolved salt,e.g., in the case of magnesium chloride where HCl was used in theacidification step, the separation of lactic acid from the magnesiumsalt solution can, e.g., be carried out by extracting the lactic acidfrom the salt solution using an organic extractant which is not misciblewith the aqueous salt solution. The lactic acid can then be recoveredfrom the extractant by, e.g., removing the solvent through evaporation,or by extracting the lactic acid from the extractant with water,resulting in the formation of an aqueous lactic acid solution.

Aqueous lactic acid solutions can be purified by methods known in theart, e.g., by treatment with active carbon. They can be concentrated byremoval of water. The lactic acid can be purified, e.g., bydistillation, resulting in a purified lactic acid. The lactic acid canbe crystallised, if so desired, to form a solid crystalline lactic acid.It can also be subjected to an oligomerisation step by removal of water,to form lactic acid oligomers.

The lactic acid obtained by the method according to the invention can beconverted to lactide. The lactide, or the lactic acid itself, can beconverted to polylactic acid. The various methods for treating themagnesium lactate, converting it to lactic acid, further treatment oflactic acid, and manufacture of lactide and polylactic acid areconventional are require no further elucidation.

The present invention is further illustrated by the following examples,without being limited thereto or thereby.

EXAMPLE 1

A magnesium lactate fermentation according to the invention was carriedout as follows. Sucrose as carbon source was brought into a fermentationreactor, together with additional nutrients and water, to form afermentation medium. The fermentation medium was brought to fermentationconditions, including a set pH and temperature. The medium wasinoculated with a microorganism capable of manufacturing lactic acid.During the fermentation, the pH of the fermentation medium wasmonitored, and kept at the selected value by the addition of a magnesiumhydroxide slurry. Sucrose as substrate was continuously added to thefermentation medium.

Periodic removal of solid magnesium lactate was carried out in such amanner that the solid magnesium lactate concentration was in the rangeof 10-40 vol. % for the entirety of the process, and in the range of15-30 vol. % for about 50% of the operating time. This was effected byremoving crystal slurry from the bottom of the reactor every 4-10 hours,removing solid magnesium lactate therefrom, and recycling the liquideffluent to the reactor.

It appeared that using the process according to the invention over aperiod of 50 hours (10 product removals) beginning at the start of thefermentation resulted in high average productivity expressed as grammagnesium lactate per liter per hour.

The magnesium lactate obtained in the process according to the inventionwas separated from the crystal slurry by filtration. The resultingfilter cake had a moisture content of between 26 and 32 wt. %. Thisindicates that the filtration properties of the magnesium lactate aresuch that efficient product separation is possible. When the solidmagnesium lactate concentration is below 5 vol. % for too long, a filtercake with a much higher moisture content will be obtained, which makesfor more difficult product separation.

The invention claimed is:
 1. Fermentation process for producingmagnesium lactate from a carbon source comprising providing afermentation medium comprising a fermentable carbon source in afermentation reactor, fermenting the fermentation medium by means of alactic acid producing microorganism in the presence of an alkalinemagnesium salt to provide a fermentation broth in the fermentationreactor comprising magnesium lactate, and recovering solid magnesiumlactate from the fermentation broth, wherein during at least 40% of atotal operating time of the fermentation process, the recovering ofsolid magnesium lactate from the fermentation broth is such that anamount of solid magnesium lactate in the fermentation broth in thefermentation reactor is maintained in a range of 5 to 40 vol. %,calculated as solid magnesium lactate in a total of the fermentationbroth in the fermentation reactor.
 2. Process according to claim 1,wherein the amount of solid magnesium lactate in the fermentation brothin the fermentation reactor is in a range of 10 to 35 vol. % during theat least 40% of the total operating time.
 3. Process according to claim1, wherein the amount of solid magnesium lactate in the fermentationbroth in the fermentation reactor is in a range of 15 to 40 vol. %during the at least 40% of the total operating time.
 4. Processaccording to claim 1, wherein the amount of solid magnesium lactate inthe fermentation broth in the fermentation reactor is in the range of 20to 40 vol. % during the at least 40% of the total operating time. 5.Process according to claim 1, wherein during at least 60% of the totaloperating time of the fermentation process, the amount of solidmagnesium lactate in the fermentation broth in the fermentation reactoris maintained in the range of 10 to 40 vol. %.
 6. Process according toclaim 1, wherein the process is a batch process, a fed-batch process, ora continuous process.
 7. Process according to claim 6, wherein theprocess is a continuous process where the amount of solid magnesiumlactate in the fermentation broth in the fermentation reactor is kept inthe range of 10 to 40 vol. % during at least 70% of the total operatingtime.
 8. Process according to claim 1, wherein the recovering solidmagnesium lactate from the fermentation broth comprises withdrawingfermentation broth comprising solid magnesium lactate from thefermentation reactor, and removing solid magnesium lactate from thewithdrawn fermentation broth.
 9. Process according to claim 8, whereinthe withdrawn fermentation broth from which solid magnesium lactate hasbeen removed is recycled back into the fermentation reactor.
 10. Processaccording to claim 1, wherein the recovering solid magnesium lactatefrom the fermentation broth is carried out in discrete steps in anintermittent fashion.
 11. Process according to claim 1, wherein therecovering solid magnesium lactate from the fermentation broth iscarried out in a continuous manner.
 12. Process according to claim 1,wherein the fermentation process is a continuous fermentation process,and wherein the recovering solid magnesium lactate from the fermentationbroth is carried out in a continuous manner.
 13. Process according toclaim 1, wherein the magnesium lactate is subjected to a purificationstep resulting in a purified magnesium lactate.
 14. Process according toclaim 1, wherein the magnesium lactate is converted to lactic acid byacidification, the acidification being followed by a separation step toseparate the lactic acid from a magnesium salt formed during theacidification step.
 15. Process according to claim 14, wherein theseparated lactic acid is subjected to one or more of a purification stepresulting in the formation of purified lactic acid, a crystallisationstep resulting in the formation of lactic acid as a solid crystallinematerial, an oligomerisation step resulting in the formation of lacticacid oligomers, or a conversion process in which the separated lacticacid is converted to lactide, or to polylactic acid either directly orvia lactide.